if it were a linestage it would be easier, put a phase splitter in front of the stage, use one stage for each phase and then possibly add another 1:1 transformer on the other end to ensure balance. But in this case you don't have analog going in, you have digital SPDIF. You can't put a phase splitter on a digital signal.
There are ways to determine a balanced signal from multple dacs with one front end, but I'm afraid it is basically a whole redesign of the DAC. If you really need a balanced out, you are better off getting a Lundahl line level transformer to split the signal. Basically take a 1674 or similar and wire it for 1:1+1 or 2:1+1. This will set you back $200+ for the pair. This may not be as advantageous as you hope, a lot depends on a lot of things in your system.
